Comedian Robin Williams was grappling with severe depression when he committed suicide Monday, and on Thursday we learned that he also was in the early stages of Parkinson's disease. Sadly, the two conditions are often found together.

Newly diagnosed Parkinson's patients have higher rates of depression, anxiety, fatigue and apathy than a control group of people without Parkinson's, according to a study published Friday in the journal Neurology.

Depression and Parkinson's are both associated with a shortage of dopamine, a neurotransmitter that helps regulate movement and control the brain's pleasure center.

"Dopamine is a feel-good chemical. If you are low in dopamine, you are not going to feel so good," said Joyce Oberdorf, president andchief executive of the National Parkinson Foundation."There are [also] other neurotransmitters that can be low."

Researchers from the Ruth and Raymond Perelman Center for Advanced Medicine at the University of Pennsylvania found that 13.9 percent of patients had symptoms of depression when they were diagnosed with Parkinson's, a proportion that rose to 18.7 percent after 24 months. Just 6.6 percent of people without the disease had depression, and that dropped to just 2.4 percent after 24 months.

Despite their depressive symptoms, most of the Parkinson's patients who also had that condition were not treated with antidepressants at any point in the two-year study.

Parkinson's is an incurable, progressive, neurological disease that afflicts about 1 million people in the United States. It affects the motor system, leaving victims with tremors, a stiff or awkward gait and repetitive, involuntary muscle movements known as dyskinesias.

In its early stages, Parkinson's is manageable through medications that increase the supply of dopamine, keep it circulating in the brain longer and make it work better, Oberdorf said.

Quality of life is often high for the first five to seven years of the disease, although it varies by individual, she said.

As the disease progresses, the ability to make dopamine and neurological receptors for the chemical declines, and symptoms often worsen. People can live for decades with Parkinson's, and the progress of the disease varies.

No one is sure what causes Parkinson's, but the thinking is that it is a combination of genetic susceptibility and environmental causes such as head trauma and exposure to pesticides.